(1) Field of the Invention
The present invention relates to a semiconductor device having a source-follower amplifier which is suitable as an amplifier device in a solid-state imaging device and the like, and a camera using the semiconductor device.
(2) Description of the Related Art
In recent years, portable terminal devices such as a cellular telephone have been equipped with digital cameras comprised of charge-coupled device (CCD) type or metal-oxide semiconductor (MOS) type imaging devices, enabling to capture still pictures and moving pictures by the digital cameras. In capturing the moving pictures, a frequency of a pixel signal is required to be high in order to consecutively capture a plurality of pictures. In capturing the still pictures, on the other hand, the frequency of the pixel signal may be low. In the case where, for example, a drive frequency is variable in a single CCD imaging device, it is desirable to minimize a frequency bandwidth of a source-follower amplifier in terms of noise reduction of the amplifier and power consumption reduction. Thus, conventional semiconductor devices have been conceived to vary a bias voltage applied to gate electrodes of load transistors in latter stages including a final stage thereby changing conductance of the load transistors to adjust a frequency bandwidth of the pixel signal (as disclosed in Japanese Patent No. 2795314 publication; pages 1-10; FIG. 5, for example).
FIG. 1 is a circuit diagram showing the conventional semiconductor device.
Referring to FIG. 1, a conventional semiconductor device 900 is comprised of source-follower amplifiers that form a three-stage structure. A source-follower amplifier in a first stage (initial stage) includes a driver transistor D1 and a load transistor L1, a source-follower amplifier in a second stage (one stage before the final stage) includes a driver transistor D2 and a load transistor L2, and a source-follower amplifier in a third stage (final stage) includes a driver transistor D3 and a load transistor L3. Note that the driver transistors D1, D2, and D3, and the load transistors L1, L2, and L3 are MOS transistors.
Drains of the driver transistors D1, D2, and D3 in the source-follower amplifiers in the respective stages are connected to a power supply terminal VDD. Sources of the load transistors L1, L2, and L3 in the source-follower amplifiers in the respective stages are connected to a ground terminal GND.
The source of the driver transistor D1 in the source-follower amplifier in the first stage is connected to the drain of the load transistor L1 and also to a gate of the driver transistor D2 in the source-follower amplifier in the second stage. A gate of the driver transistor D1 is connected to an input terminal Vin to which a pixel signal is inputted in capturing moving pictures and still pictures.
The source of the driver transistor D2 in the source-follower amplifier in the second stage is connected to the drain of the load transistor L2 and also to a gate of the driver transistor D3 in the source-follower amplifier in the third stage.
The source of the driver transistor D3 in the source-follower amplifier in the third stage is connected to the drain of the load transistor L3 and also to an output terminal Vout from which the pixel signal is outputted to the outside.
Furthermore, a gate of the load transistor L1 in the source-follower amplifier in the first stage is connected to the ground terminal GND, while gates of the load transistors L2 and L3 in the source-follower amplifiers in the second and third stages are applied with a same bias voltage LG that can vary to achieve a desirable frequency bandwidth of the signal inputted into the input terminal Vin. This enables the frequency bandwidth and the amount of power consumption to be consecutively changed by varying the bias voltage LG.
Here, in the source-follower amplifier in a stage, a transconductance gmD of a driver transistor D is determined by the following equation, wherein a mobility is μD, a gate width is WD, a gate length is LD, a gate capacitance is CoxD, a threshold voltage is VtD, an input voltage is Vi, and an output voltage is Vo, regarding the driver transistor D:gmD=μD*CoxD*(WD/LD)*(Vi−Vo−VtD).  (1)
Further, a bandwidth ft defined as a frequency whose amplification factor is decreased by 3 dB than an amplification factor of a direct current component is determined by the following equation, wherein a load capacitance of the signal outputted from the source-follower amplifier is C:ft=2*n*gmD/C.  (2)
Note that the load capacitance C includes an input capacitance of the driver transistor in the next stage, a wiring capacitance, and the like. Note also that the source-follower amplifier in the final stage is connected to an external circuit thereby increasing its load capacitance more than load capacitances in other stages.
Furthermore, a consumption current I depends on a load transistor L serving as a constant current source, and determined by the following equation, wherein a mobility is μL, a gate width is WL, a gate length is LL, a gate capacitance is CoxL, a threshold voltage is VtL, and a gate-source voltage is VgsL, regarding the load transistor L:I=(1/2)*μL*CoxL*(WL/LL)*(VgsL−VtL)2.  (3)
Still further, a gain G is determined by the following equation, wherein a transconductance of a back gate of the driver transistor D is gmb, a conductance of the driver transistor D is gdsD, and a conductance of the load transistor L is gdsL:G=gmD/(gmD+gmb+gdsD+gdsL).  (4)
Still further, the gain G is also determined by the following equation, wherein the equations (1) and (3) are assigned to the equation (4), and a coefficient of WD in the numerator is a and a coefficient of WL in the denominator is c:G=(c·WD)/(a*WD+b*WL).  (5)
Note that an operating point of the output voltage Vo of the source-follower amplifier depends on a resistance ratio between the driver transistor D1 and the load transistor L1.
In general, the source-follower amplifiers are used in a multi-stage structure in a signal outputting unit to reduce an output impedance.
In such multi-stage source-follower amplifiers, since the load capacitances are increased gradually towards the final stage, the WD and LD of the driver transistors D are expanded gradually towards the final-stage thereby increasing the gmD in the equation (1), which adjusts to make the frequency bandwidth by the equation (2) appropriately constant (constant-bandwidth technology).
However, in the above case, when only the WL and LL of the drive transistor D are expanded, the operating points are varied, and thereby, the WL and LL of the load transistor are expanded as well. As a result, the consumption current is greater than the consumption current calculated by the equation (3), and it should be noted that the consumption currents are greater in the latter stage.
Thus, when still pictures are captured, it is efficient to save unnecessary electric currents in the latter stages.
Moreover, there is another example of the method for adjusting the frequency bandwidths and the consumption currents by varying gate bias voltages applied to the load transistors in the latter stages in order to reduce the consumption currents.
The equation (3) shows that the consumption current can be changed by varying the gate bias voltage. The equation (2) shows that the frequency bandwidth can be changed by changing the consumption current thereby varying the operating point Vo and eventually varying the transconductance. In this case, both the gain and the operating point are changed, and the amount of change can vary within an acceptable range, but the amount of change is small as described further below with reference to FIGS. 2 to 5 so that the gain and the operating point can vary within a substantially wide range. Furthermore, regarding linearity of input-output characteristics, a linear region of the input-output characteristics is reduced when the operating point is too low or too high, but the operating point of the multi-stage amplifiers is generally getting lower gradually towards the latter stages, so that, when voltages applied to the gates of the load transistors in the latter stages can vary, it does not need to consider a limit of a linearity whose operating point is high. A limit of a linearity whose operating point is low can vary within a substantially wide range since the change of the operating point is small as described above. Therefore, the three-stage source-follower amplifiers, in which the frequency bandwidths are reduced, enable a reduction in the consumption current compared to the consumption current in the two-stage source-follower amplifiers.
This results from that in the three-stage source-follower amplifiers, since the load capacitance in the second stage, which is not the final stage, decreases, the consumption current can be reduced compared to the case where the second stage is the final stage. This is because the load capacitance in the last stage, that is directly affected by the external capacitance, is relatively greater.
This also results from that, as described for the constant-bandwidth technology, in the three-stage source-follower amplifiers, the transconductance of the driver transistor in the third stage is greater than the transconductance of the driver transistor in the second stage, that is, the frequency bandwidth of the calculated by the equation (2) is greater than that of the two-stage source-follower amplifiers, so that the consumption currents can be reduced for the amount of the resulting extra currents.
Here, characteristics of the conventional semiconductor device 900 are examined. The gate widths and the gate lengths of the respective driver transistors are 8 μm and 4 μm in the first stage, 80 μm and 4 μm in the second stage, and 800 μm and 4.5 μm in the third stage, respectively, while the gate widths and the gate lengths of the respective load transistors are 10 μm and 26 μm in the first stage, 110 μm and 10 μm in the second stage, and 150 μm and 10 μm in the third stage, respectively.
When the gate bias voltage LG applied to the load transistors in the second and third stages varies from −5 V to 0 V, a linear region of input characteristics is shown in FIG. 2, the consumption current in the second stage is shown in FIG. 3, the consumption current in the third stage is shown in FIG. 4, and a frequency decreased by 3 dB is shown in FIG. 5.
Referring to FIG. 5, the frequency decreased by 3 dB is 43 MHz when the LG is −5V, and 145 MHz when the LG is 0V. In this case, the consumption currents are 1 mA and 10 mA, respectively, and the gain and operating points vary within an acceptable range. Furthermore, the consumption current in the third stage is reduced from 5 mA to 1 mA in the same bandwidth as compared to the characteristics in the two-stage source-follower amplifiers.
In recent years, semiconductor devices have been developed to increase the number of pixels in the same dimensions so that a frequency of the pixel signal has also been increased as the increase of the number of pixels in capturing moving pictures and the like.
However, the conventional semiconductor device can adjust the frequency bandwidths and the consumption currents in the latter stages, but cannot do so in the initial stage, which fails to adjust the frequency bandwidths and the consumption currents in all stages. As a result, if the frequency bandwidth in the initial stage has previously been adjusted to capture moving pictures by the usual number of pixels, the conventional semiconductor device cannot properly transmit a pixel signal when the frequency of the pixel signal is increased to capture moving pictures by the increased number of pixels. Conversely, if the frequency bandwidth in the initial stage has previously been adjusted to a high frequency to capture moving pictures by the increased number of pixels, a lot of the consumption currents are wasted in the source-follower amplifier in the initial stage in capturing moving pictures by the usual number of pixels and in capturing still pictures.
Thus, this causes a problem that the conventional semiconductor device cannot appropriately adjust the frequency bandwidths and the consumption currents to be increased or reduced in the source-follower amplifiers in all stages.